The long-term goal of this research is to apply novel technology for detection of donor organ (allograft) 3 rejection to improve patient outcomes following heart transplantation. The specific goal of this study is to 4 determine whether daily monitoring of the transplant recipient's electrocardiogram (ECG) using a simple home 5 device with transmission to an ECG Core Laboratory would provide an early biomarker for acute rejection. 6 Despite routine immunosuppressant drug therapy, acute rejection is common, especially within the first 6 7 months following transplant surgery. To detect rejection, frequent endomyocardial biopsies of heart tissue are 8 performed. An endomyocardial biopsy is a costly and invasive procedure performed in a hospital cardiac 9 catheterization laboratory that has associated risks. Recent evidence suggests that acute allograft rejection 10 causes delays in ventricular repolarization resulting in a longer QT interval on the ECG. The specific aims of 11 the study are to: 1) determine whether an increase in the QT interval during the first 6 months following heart 12 transplant is a sensitive and specific biomarker for acute rejection; and 2) determine the timing of QT interval 13 increases relative to biopsy-diagnosed stages of mild/moderate/severe rejection. Secondary aims are to: 3) 14 determine whether an increase in the QT interval predicts 1-year mortality; and 4) explore additional ECG 15 measurements that might predict rejection or death. 16 A prospective, double-blind, descriptive and correlational research design is planned. Investigators who 17 analyze ECG data will be blinded from biopsy results; healthcare providers will be blinded from QT 18 measurements. A sample of 325 adult heart transplant recipients will be recruited from 3 transplant centers: 19 Columbia University-New York Presbyterian Medical Center; University of California, Los Angeles and Cedars- 20 Sinai Medical Center, Los Angeles. Each subject will record a 30-second ECG daily using the HeartOneTM 21 device; the device will automatically transmit the ECG to an ECG Core Laboratory at the University of 22 California, San Francisco. For Primary Aim #1, a log regression analysis will be performed with the 23 independent variable being presence /absence of the ECG criteria ( QTC e25 milliseconds X 3 days) and the 24 dependent variable being presence /absence of acute allograft rejection. Odds ratios and confidence intervals 25 will be reported, as well as established evaluative criteria for new diagnostic instruments (sensitivity, specificity, 26 positive /negative predictive value, predictive accuracy). For Primary Aim #2, descriptive statistics will be used 27 to report the average time period between the development of the ECG criterion and biopsy evidence of 28 rejection. The potential benefit of finding a simple ECG biomarker of allograph rejection that could be 29 measured at home is that it might yield earlier detection of rejection, allow more timely therapy and reduce 30 mortality from acute allograft rejection.